1. Field of the Invention
The present invention relates to a structure for mounting a magnet. Specifically, the invention relates to a magnet mounting structure for attaching a magnet which renders a signal to a sensor to detect the running speed of a two-wheeled vehicle.
2. Description of the Background Art
An increasing number of bicycles employ a speedometer mounted thereto for facilitating ascertainment of the travelling speed, travelling distance and the like of the bicycles. FIG. 5 is an external view of a bicycle incorporating a speedometer therein and FIG. 6 is an enlarged view of the portion indicated by X in FIG. 5. A signal generator/transmitter portion is constituted of a transmitter unit 128 detachably mounted to a front fork 134 of a bicycle 124 and a magnet 113 detachably mounted to a front spoke 130 thereof. Magnet 113 is spaced from a front axis 136 by a distance A corresponding to the distance between front axis 136 and transmitter unit 128.
These components are thus structured so that each time magnet 113 passes in front of transmitter unit 128, a magnetic flux originated from magnet 113 causes contacts of a reed switch (not shown) incorporated in transmitter unit 128 to close and accordingly a signal of one pulse is generated. The generated signal is transmitted from transmitter unit 128 to a receiver unit 135 attached to a stem 119. Receiver unit 135 counts the transmitted signal to calculate various measurements such as the travelling speed, travelling distance and the like of the bicycle. A display unit (not shown) is further provided formed of liquid crystal for example for indicating calculated measurements.
Although the magnet is attached to a single spoke as shown in FIG. 6, the magnet may be attached to two spokes. FIG. 7 shows a conventional structure for mounting a magnet to a single spoke 130. This mounting structure is used to attach the magnet in the following manner. First, the spoke is inserted through two slits cut in a projecting piece 103 jutting out from a magnet housing 102. The slits extend from top end to magnet housing 102. Then, a ring 104 is fit on projecting piece 103 and a setscrew 105 is screwed into a threaded portion 108 provided inside projecting piece 103. As setscrew 105 is being screwed in, ring 104 is pushed toward the spoke by the head of setscrew 105. Ring 104 in turn pushes spoke 130 toward magnet housing 102 while keeping contact with spoke 130 at two spots. Finally, ring 104 and magnet housing 102 are secured to spoke 130 with spoke 130 held therebetween. Ring 104 keeps pressing spoke 130 while contacting spoke 130 at the two spots. In this way, a firm attachment is achieved. For screwing setscrew 105, a tip of a screwdriver is fit in a driver slot made in the head of setscrew 105 shown in FIG. 7 to turn screw 105. Ring 104 of this securing structure restrains the perimeter of projecting piece 103 from bending outward. Then, the spoke in the slits can be engaged firmly with the slits to ensure the attachment.
However, the magnet mounting structure as described above requires some tool such as Phillips screwdriver, flatblade screwdriver or the like for mounting the magnet to the spoke. Unless such a tool is used for screwing, the magnet mounting structure cannot be secured firmly to the spoke. In other words, the conventional magnet cannot be secured to the spoke unless there is such a tool at hand.